Steve Thompson wrote:
>Ian White GM3SEK wrote:
>> Let's think of two totally generalized impedances, (R1 in series with
>> jX1) and (R2 in series with X2). In this context X1 and X2 can have
>> either sign.
>> Now add series elements -X1 and -X2 to cancel both reactances out. We
>> now have resistive impedances R1 and R2, which can be matched with the
>> minimum-Q solution using two elements in one of the L-network
>This is where I started - but you can always add a first element to take
>you to a pure resistance that's nearer your target than (say) R1 is.
>Then, the L match from the new resistance to target will have a lower Q
>than the one from R1 to target and the three elements give you lower Q
>I think the train of thought can be valid where you can't choose
>whichever of the L match permutations is needed for minimum Q - that's
>the trap I fell in to.
That's a fair point. The more elements you add, the more possibilities
you have to synthesize a network that transforms the impedance in a
cascade of small steps. Then, as Peter points out, it begins to look a
lot like a filter... and disappears into a cloud of complex maths.
In practical terms, a very nice thing to do with a large T-tuner is the
mod suggested by G3LNP: fit a cam switch to each capacitor, so that
rotating it to maximum capacitance will short the cap completely. This
gives the user an instant choice between a T-network, a step-up
L-network or a step-down L-network. Tune for minimum smoke.
73 from Ian GM3SEK
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